(1) Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating it, and more particularly to a structure of a capacitor insulating film and a method for fabricating it.
(2) Description of the Prior Art
Conventionally, as capacitor insulating films, there are used a silicon oxide film (SiO.sub.2), a silicon nitride Film (Si.sub.3 N.sub.4) and a composite film of these films (e.g., a double layered film of SiO.sub.2 /Si.sub.3 N.sub.4, film of SiO.sub.2 /Si.sub.3 N.sub.4 /SiO.sub.2). FIG. 1 shows the structure of one example of these conventional films. In FIG. 1, a lower electrode 3 having a predetermined shape and made of impurity-doped polysilicone (also called polycrystalline silicon) is formed on the surface of the silicon oxide film 5 on a semiconductor substrate 6. A silicon nitride film 2 having a thickness of about 10 nm is formed by chemical vapor deposition techniques so as to cover the surface of the lower electrode 3. Thereafter, the surface of the silicon nitride film 2 is thermally oxidized to be converted into a silicon oxide film 7 having a thickness of about 2-3 nm. Subsequently, an upper electrode 4 made of polysilicon is formed so as to cover the silicon oxide film 7. In the above structure, the capacitor insulating film is composed of the silicon nitride film 2 formed by the chemical vapor deposition and the silicon oxide film 7 formed by the thermal oxidation. The capacitor insulating film has a thickness of about 7-8 nm in equivalent thickness of an oxide film. Reference is made to "Y. Ohji et al., Proceedings of 25th Annual IEEE Reliability Physics Symposium (1987) p. "55
The conventional capacitor insulating film has the Following defects. The conventional silicon nitride film formed by the chemical vapor deposition contains many weak-spots so that a single silicon nitride film of 8 nm or less presents difficulties in its use as a stabilized capacitor insulating film with less leakage current. For this reason, a double layered SiO.sub.2 /Si.sub.3 N.sub.4 film wherein the weak spots are covered by oxidation film on tile surface of the silicon nitride film is now widely used in the current DRAMs, etc. However, this double layered film, when it has a thickness of, 5 nm or less in equivalent thickness of an oxide film, also suffers from an abrupt increase in the leakage current. Reference is made to "J. Yugami et al., Extended Abstracts on the 20th Conference on Solid State Devices and Materials (1988) p.173"
Further, the chemical vapor deposition tends to introduce oxygen into the silicon nitride film. This introduction of oxygen is remarkable particularly in the initial process where the silicon nitride film grows on the oxide film. Also where a silicon nitride film is to be grown on polysilicone from a vapor phase, it will actually grow on the native oxide film formed on the polysilicone surface. Thus, it was difficult to form a stoichiometric silicon nitride film 10 nm or less thick on polysilicon by the chemical vapor deposition techniques. Reference is made to "J. A. Wurzbach and F. J. Grunthaner, J. Electrochem. Soc., Vol 130, No.3 (1983) p.691" .